It is well known that polymeric polyesters prepared by polycondensation of a dihydric alcohol or its functional derivatives and a dicarboxylic acid or a polyester forming derivative thereof such as an acid halide, or a simple diester of a dibasic acid and a volatile monohydric alcoholic, are excellent fiber forming or moldable polymers.
Commercially, the most important polyesters are those prepared by the condensation of terephthalic acid or dimethylterephthalate and a polymethylene glycol containing from 2 to 10 carbon atoms, and more particularly ethylene glycol, 1,4-cyclohexandeimethanol or mixtures thereof.
These polyesters are relatively inert and hydrophobic materials which are capable of being formed into filaments which can be drawn to produce textile fibers of superior strength and pliability.
Unfortunately, because of their compact molecular structure, polyesters, and in particular polyethylene terephthalate, are difficult to dye or otherwise color after the polymer has been formed into a fiber or molded shape.
As a result, a substantial body of art has developed in the modification of polyesters to improve their receptivity to dyes and color modifying compounds.
A desirable feature of polymer materials in many instances is an appearance of whiteness. For instance, it is often desired that a fabric manufactured from polyester fiber present the appearance of whiteness to the observer. Unfortunately, native polyester fiber as manufactured has a yellowish appearance unacceptable to the observer.
At the present time, in order to improve the apparent whiteness in polyester materials, toners are incorporated into the polyester to hide the yellow color. Such toners usually have a visible absorption maximum in the range of 575 nm to 595 nm as measured in acetone.
Cobalt acetate is one of the most widely used toners in the industry to mask the yellow color of polymers. However, cobalt acetate has a number of disadvantages.
For instance, cobalt acetate toner materials tend to be unstable during storage as the result of temperature and humidity, and undergo an undesirable color shift toward yellow. Hence, when high cobalt concentrations are needed to mask the yellow color of some polymers there is a tendency to impart a grey color to the polymer.
Another disadvantage is the limitation by certain regulatory bodies of the level of cobalt usable in polyester catalyst systems.
In addition, cobalt lowers polymer thermal stability and increases acetaldehyde formation in poly(ethylene terephthalate).
Lastly, cobalt has a strong tendency to form insoluble residues in manufacturing process equipment, leading to manufacturing quality control problems.
In addition to overcoming the disadvantages of cobalt acetate as a toner for polyesters, an improved toner must be stable under the conditions of polymerization and melt processing. The toner must be stable to light and to other environmental conditions to which the fiber, fabricated article prepared therefrom may be exposed. An improved toner must be readily incorporated into a polyester without deleteriously affecting the physical properties of the polymer.
Thus, there still is a continuing need for polyesters having improved whitness.